Starship On-orbit refueling - Options and Discussion

[docmordrid]

I can't help but think SpaceX may leverage Tesla's "snake" combined with a quick connect/disconnect....

[TheRadicalModerate]

For the capture and docking process you need somewhere to put the mechanism.

You're going to have 6 dirty great cargo pods inside the skirt area.....
Why not use part of that volume for the docking mechanism?

With six equally spaced around the circumference you can have 3 'male' and 3 'female' mechanisms.
When 2 SS's are tail to tail this gives you the 180^o relative position we've seen in all renders for fueling so far.

There are only 3 cargo pods, if they still exist at all.  The other 3 spots have RVacs in them.  But less is more: you only need enough to be able to stabilize and control both Starships in all six degrees of freedom.

Three capture devices work fine, with the arrangement shown, assuming that there are both fill and drain lines for both LCH4 and LOX:

[paulthew]

Has anybody proposed using a heat pump to make fuel transfers in space happen?  If two tanks in zero gravity are connected and they are different temperatures, all of the fuel should end up in the cooler tank.  The greater the heat difference, the quicker the transfer.

[SkyRate]

Has anybody proposed using a heat pump to make fuel transfers in space happen?  If two tanks in zero gravity are connected and they are different temperatures, all of the fuel should end up in the cooler tank.  The greater the heat difference, the quicker the transfer.

Two big assumptions: That you can heat it a lot and that you can contain the pressure that that creates. Venting the gas out of the target tank + optionally pumping is way safer.

[livingjw]

Has anybody proposed using a heat pump to make fuel transfers in space happen?  If two tanks in zero gravity are connected and they are different temperatures, all of the fuel should end up in the cooler tank.  The greater the heat difference, the quicker the transfer.

Two big assumptions: That you can heat it a lot and that you can contain the pressure that that creates. Venting the gas out of the target tank + optionally pumping is way safer.

No heating or venting is required. Using a low pressure electric pump allows for reasonable transfer rates without venting.

John

[tbellman]

I tend to agree that the feet are going to be dicey to use for this.  I also agree that repurposing the QDs for use as the hard-dock mechanism makes sense, especially since the hard-dock doesn't need to hold pressure (except in the fueling lines themselves, of course).

Just to make sure we are not talking past each other:  In my post, in the parts where I used the words "quick disconnect" (or "QD"), I was specifically talking about connectors for the propellant lines.  The mating mechanism for holding together SuperHeavy and Starship during launch are of course also a kind of quick disconnect, and I should probably have been clearer in my wording and used "prop QD" or something like that when I was talking about the propellant lines.

And I think it is very unlikely that the propellant line QDs will be used for holding the ships together.  Rather, they would only connect and engage after the two ships are securely clamped together by the docking mechanisms, and all relative motions have been cancelled.  They would also be designed to disconnect, close their valves and retract when subject to fairly small forces, so as to minimize risk of rupturing the propellant lines.

I suspect that in your post, when you were talking about repurposing the QDs for hard docking, you were talking about the mechanisms for securely holding together SH and SS, right?

As for the soft capture part of docking, I don't have any insights or good guesses on how that will be handled.  Other than that it could probably be useful during the stacking of a Starship on top of SuperHeavy on the ground as well; they also need to be aligned properly.

[rakaydos]

Has anybody proposed using a heat pump to make fuel transfers in space happen?  If two tanks in zero gravity are connected and they are different temperatures, all of the fuel should end up in the cooler tank.  The greater the heat difference, the quicker the transfer.

Two big assumptions: That you can heat it a lot and that you can contain the pressure that that creates. Venting the gas out of the target tank + optionally pumping is way safer.

Doesnt that requir a return line? I thought SpaceX was aiming to just have 1 transfer line (also used for initial fueling) and a vent line, to save weight
No heating or venting is required. Using a low pressure electric pump allows for reasonable transfer rates without venting.

John

[Slarty1080]

For the capture and docking process you need somewhere to put the mechanism.

You're going to have 6 dirty great cargo pods inside the skirt area.....
Why not use part of that volume for the docking mechanism?

With six equally spaced around the circumference you can have 3 'male' and 3 'female' mechanisms.
When 2 SS's are tail to tail this gives you the 180^o relative position we've seen in all renders for fueling so far.

There are only 3 cargo pods, if they still exist at all.  The other 3 spots have RVacs in them.  But less is more: you only need enough to be able to stabilize and control both Starships in all six degrees of freedom.

Three capture devices work fine, with the arrangement shown, assuming that there are both fill and drain lines for both LCH4 and LOX:

I don't think this will work. They need to have bilateral symmetry. Starships will approach each out back to back and inverted so the heatshields of each will be on different sides

[rakaydos]

For the capture and docking process you need somewhere to put the mechanism.

You're going to have 6 dirty great cargo pods inside the skirt area.....
Why not use part of that volume for the docking mechanism?

With six equally spaced around the circumference you can have 3 'male' and 3 'female' mechanisms.
When 2 SS's are tail to tail this gives you the 180^o relative position we've seen in all renders for fueling so far.

There are only 3 cargo pods, if they still exist at all.  The other 3 spots have RVacs in them.  But less is more: you only need enough to be able to stabilize and control both Starships in all six degrees of freedom.

Three capture devices work fine, with the arrangement shown, assuming that there are both fill and drain lines for both LCH4 and LOX:

I don't think this will work. They need to have bilateral symmetry. Starships will approach each out back to back and inverted so the heatshields of each will be on different sides

Why inverted? Back to back is enough to flip connection points, with, say, a male connector on the left and a female connector on the right.

[Pete]

About propellant needs for settling the prop and transfer.
Surely once you are actually pumping the liquids across, the rather massive shift in center of mass will remove the need for any further sustained ullage thrust? You are moving half a ton of mass over a distance of more than 30 meters, *per second*. This will impart a very significant apparent "thrust" to the tank being pumped out, resulting in all the settling you could possibly need.

[rsdavis9]

For the capture and docking process you need somewhere to put the mechanism.

You're going to have 6 dirty great cargo pods inside the skirt area.....
Why not use part of that volume for the docking mechanism?

With six equally spaced around the circumference you can have 3 'male' and 3 'female' mechanisms.
When 2 SS's are tail to tail this gives you the 180^o relative position we've seen in all renders for fueling so far.

There are only 3 cargo pods, if they still exist at all.  The other 3 spots have RVacs in them.  But less is more: you only need enough to be able to stabilize and control both Starships in all six degrees of freedom.

Three capture devices work fine, with the arrangement shown, assuming that there are both fill and drain lines for both LCH4 and LOX:

I don't think this will work. They need to have bilateral symmetry. Starships will approach each out back to back and inverted so the heatshields of each will be on different sides

Why inverted? Back to back is enough to flip connection points, with, say, a male connector on the left and a female connector on the right.

Maybe because you need to mate CH4 to CH4 and LOX to LOX? Which maybe on opposite sides of the ship.

[Slarty1080]

About propellant needs for settling the prop and transfer.
Surely once you are actually pumping the liquids across, the rather massive shift in center of mass will remove the need for any further sustained ullage thrust? You are moving half a ton of mass over a distance of more than 30 meters, *per second*. This will impart a very significant apparent "thrust" to the tank being pumped out, resulting in all the settling you could possibly need.

The problem is each of those half ton masses is going to impart and acceleration forward, but then it will hit the reverse bulkhead or liquid surface and impart an equal but opposite acceleration in the opposite direction. If they could get any useful acceleration from just pumping liquid around a ship they would use it to propel the ship.

[tbellman]

About propellant needs for settling the prop and transfer.
Surely once you are actually pumping the liquids across, the rather massive shift in center of mass will remove the need for any further sustained ullage thrust? You are moving half a ton of mass over a distance of more than 30 meters, *per second*. This will impart a very significant apparent "thrust" to the tank being pumped out, resulting in all the settling you could possibly need.

The problem is each of those half ton masses is going to impart and acceleration forward, but then it will hit the reverse bulkhead or liquid surface and impart an equal but opposite acceleration in the opposite direction. If they could get any useful acceleration from just pumping liquid around a ship they would use it to propel the ship.

No, Pete is correct.  As the propellant is moved (pumped) in one direction, everything else, i.e. the ships themselves, must shift in the opposite direction.  Centre of mass of a system (in this case ships+propellant) cannot move, unless there is some external force acting on it.

As soon as you stop pumping, though, the ships will also stop moving.  The end result will then be that the propellant has moved some distance in the -X direction (relative to the tanker) while the ships' hulls have moved some distance in the +X direction.  The centre of mass for the entire system will not have moved at all, however, which is why you can't use pumping for propulsion.

(And movement here is relative to an observer in the same trajectory as the ships.)

[cdebuhr]

About propellant needs for settling the prop and transfer.
Surely once you are actually pumping the liquids across, the rather massive shift in center of mass will remove the need for any further sustained ullage thrust? You are moving half a ton of mass over a distance of more than 30 meters, *per second*. This will impart a very significant apparent "thrust" to the tank being pumped out, resulting in all the settling you could possibly need.

The problem is each of those half ton masses is going to impart and acceleration forward, but then it will hit the reverse bulkhead or liquid surface and impart an equal but opposite acceleration in the opposite direction. If they could get any useful acceleration from just pumping liquid around a ship they would use it to propel the ship.

No, Pete is correct.  As the propellant is moved (pumped) in one direction, everything else, i.e. the ships themselves, must shift in the opposite direction.  Centre of mass of a system (in this case ships+propellant) cannot move, unless there is some external force acting on it.

As soon as you stop pumping, though, the ships will also stop moving.  The end result will then be that the propellant has moved some distance in the -X direction (relative to the tanker) while the ships' hulls have moved some distance in the +X direction.  The centre of mass for the entire system will not have moved at all, however, which is why you can't use pumping for propulsion.

(And movement here is relative to an observer in the same trajectory as the ships.)

I had Pete's very same tough yesterday, and nearly posted about it, but I couldn't shake the feeling that I was missing something important.  Now I think I see the problem.  While its true that moving prop will shift the COG, resulting in overall movement of the mated ships, that alone isn't good enough.  To ensure that fluids stay settled, the ships need to be continuously accelerating, otherwise any deviation from a perfectly smooth transfer risks bumping the fluids away from the lower bulkhead.  Unless you're actually ramping the pumping rate over time, this requirement would not be met.  Now I suppose you could ramp the pumping rate - whether or not that would be enough to make this work, I won't comment on, as I've not done the math, but it's slightly more complicated than what was initially proposed.

[TheRadicalModerate]

I don't think this will work. They need to have bilateral symmetry. Starships will approach each out back to back and inverted so the heatshields of each will be on different sides

It does have bilateral symmetry--about the y-axis.  Note that the second image hasn't been folded to be back-to-back yet.  When you do that, everything matches up.

But the reason I had separate fill and drain lines is that I couldn't get a double rotation-invariant version to work without them.  My problem was that I hadn't put them on the axis of symmetry.  When I do that, I get this:

Update:  Note that there's no particular reason whey the soft captures can't hug the inner mould line and have the LCH4 line be coaxial with it, other than I suspect you really want that sucker welded to the wall.  If that's the case, than the capture device needs to be mounted inboard of it.  I suspect either way can be made to work.

[Apollo-phill]

Just side tracking but didn't Soviets/Russians undertake propellant transfer during some early dual flight Soyuz spacecraft  missions ?

And, if so, any published data/results on what was achieved and, any relevance to this topic ?

Also, may have been noted earlier - why transfer propellant from ship to ship ?
Why not undock crew/payload stage/module and just dock with the  upcoming "propellant 'tanker' " which then becomes the "celestial target bound" starship ? The now , payload less , empty starship returns to landing site. Just a thought.

Phill

[TheRadicalModerate]

Just side tracking but didn't Soviets/Russians undertake propellant transfer during some early dual flight Soyuz spacecraft  missions ?

And, if so, any published data/results on what was achieved and, any relevance to this topic ?

Also, may have been noted earlier - why transfer propellant from ship to ship ?
Why not undock crew/payload stage/module and just dock with the  upcoming "propellant 'tanker' " which then becomes the "celestial target bound" starship ? The now , payload less , empty starship returns to landing site. Just a thought.

Phill

The Russians transfer hydrazine and NTO using a bladder that lines the inside of the tank.  As the bladder contracts, it provides the pressure to move the prop from one tank to another.

For many Starship missions, you need more propellant than a tanker can haul up in a single launch, so swapping payload sections doesn't get the job done.

[RDoc]

Just side tracking but didn't Soviets/Russians undertake propellant transfer during some early dual flight Soyuz spacecraft  missions ?

And, if so, any published data/results on what was achieved and, any relevance to this topic ?

Also, may have been noted earlier - why transfer propellant from ship to ship ?
Why not undock crew/payload stage/module and just dock with the  upcoming "propellant 'tanker' " which then becomes the "celestial target bound" starship ? The now , payload less , empty starship returns to landing site. Just a thought.

Phill

My understanding is that there will be several tanker loads per flight and Starship doesn't really have a separate payload stage, it's all one piece.
Transferring the payload(s) in orbit sounds pretty tricky compared to pumping liquid in any case.

[DistantTemple]

Just side tracking but didn't Soviets/Russians undertake propellant transfer during some early dual flight Soyuz spacecraft  missions ?

And, if so, any published data/results on what was achieved and, any relevance to this topic ?

Also, may have been noted earlier - why transfer propellant from ship to ship ?
Why not undock crew/payload stage/module and just dock with the  upcoming "propellant 'tanker' " which then becomes the "celestial target bound" starship ? The now , payload less , empty starship returns to landing site. Just a thought.

Phill

It takes several tankers to "fill" an SS sufficiently for all expected tasks... except LEO perhaps. So orbiting and lift tankers (same or different) would still have to transfer fuel to prepare one, as you say "celestial target bound" starship"
Edit; great this all answering at once! ;-(

[rakaydos]

The problem with using tranfereed fuel CoM shift as ullage, is that the COM movement is measured in M/s, and ullage is measured in M/s/s. Once the fuel is being transfered at a steady rate, you no longer get any acceleration, just a steady shift in CoM.